1. Field of the Invention
This invention relates to a variable-capacitance type sensor acting as a pressure-capacitance transducer and used in a pressure sensor and the like and to a variable-capacitance type sensor system for sensing variation in the electrical capacitance of the variable-capacitance type sensor.
2. Description of the Related Art
For example, devices such as pressure sensors may be effectively used in refrigeration systems mounted on cars so as to sense the pressure of the refrigerant, convert the sensed pressure into an electrical signal, and supply the same to a control circuit.
A diaphragm formed of a silicon semiconductor is known as the above pressure sensor. However, when the pressure sensor is used under a severe mounting condition and a violent vibration condition like the devices mounted on a car, a problem tends to occur in the conditions encountered by durability and reliability of the pressure sensor. With such a problem taken into consideration, a variable-capacitance type transducer has been provided as a pressure sensor which can be most adequately used in the above severe service condition. The variable-capacitance type transducer is disclosed in, for example, U.S. Pat. No. 4,716,492 by Charboneau et al. When a distance between two electrode plates constituting a capacitor varies according to pressure, the variable-capacitance type transducer derives the variation in the distance as a variation in the capacitance of the capacitor. Then, the derived capacitance is compared with the capacitance of a reference capacitor to provide an electrical signal corresponding to the pressure.
However, the variable-capacitance type transducer only has a pressure sensing range of 100 Kpa to 5 Mpa. Therefore, in a case where the pressure sensor is used as a pressure sensing means of the refrigeration system of the car, it is necessary to provide an electric circuit for effecting the signal processing to derive an electrical signal representing variation in the capacitance of the variable-capacitance type transducer at a distance from the pressure sensing terminals of the transducer because of the severe service condition in addition to the narrow pressure sensing range. However, if an electric circuit disposed at a distance from the pressure sensing terminals is used to effect the signal processing, the S/N ratio becomes extremely low and it becomes impossible to derive an effective signal.
In order to solve the above problem in the prior art, a signal processing system for converting variation in the capacitance into a corresponding voltage signal is disposed near the pressure sensing terminals of the transducer as is disclosed in U.S. patent application No. 85961 filed by Kawate et al. on May 5, 1986 and now abandoned. That is, the sensor of Kawate et al. is constructed such that a variable capacitor is disposed on the under surface of a ceramic base plate in the sensor and a signal processing circuit for converting a capacitance variation signal into a voltage signal and an electric circuit for sensor calibration are disposed on the upper surface of the base plate and are integrally formed with the variable capacitor in a compact form. With the above sensor, when the variable capacitor primarily generates an electric signal in response to a physical amount such as pressure to be measured, the electric signal is converted into a voltage signal by means of the signal processing circuit and at the same time subjected to a calibration process by means of the electric circuit for calibration so as to derive a voltage signal necessary for the control. That is, with the sensor of Kawate et al., an electric signal of high quality or voltages of practically wide range can be obtained by disposing the signal processing circuit which can be formed in an IC form and the electric circuit for sensor calibration extremely near the sensing terminals of the pressure sensor.
However, when the construction in which the signal processing circuit and the electric calibration circuit are disposed extremely near the sensing terminals of the pressure sensor is actually applied to a sensor system, the following undesirable problems will occur.
That is, since the transducer is generally disposed near an object to be measured, it tends to be influenced by the electromagnetic field in the environment in which the object is disposed. Therefore, in order to derive a reliable signal, it is necessary to form the transducer with a construction which is hardly affected by the influence of the measuring environment. However, in the sensor system described above, the reference capacitor and the variable-capacitance capacitor are selected together as one of the two sets of different power source potentials by means of a complicated operation system including a selection switch, thus permitting the sensor system to be operated on an intermediate potential. Therefore, it becomes necessary to separate a pair of opposing electrodes of the transducer from a portion which is set at the ground potential. As a result, a stray static capacitance occurs between the electrodes and the object to be measured and the capacitance thereof tends to be influenced by a variation in the external electric field. Further, the variable-capacitance capacitor is discharged via the stray static capacitor formed between the variable-capacitance capacitor and the ground, causing a measurement error.